1. Field of the Invention
This invention relates to an evaporative emission control system for internal combustion engines, which operates to prevent evaporative fuel from being emitted from a fuel supply system of the engine, and more particularly to an evaporative emission control system for an internal combustion engine which is equipped with a so-called exhaust secondary air supply system.
2. Prior Art
Conventionally, internal combustion engines equipped with a so-called exhaust secondary air supply system are well known, which system supplies secondary air into an exhaust pipe of the engine in which is provided a catalyst typically represented by a three-way catalyst. The exhaust secondary air supply system supplies secondary air into the exhaust pipe at a zone upstream of the catalyst when the air-fuel ratio of a mixture supplied to the engine is rich with respect to a stoichiometric air-fuel ratio so that exhaust gases are burned at the stoichiometric air-fuel ratio to thereby obtain the best conversion efficiency of the catalyst.
On the other hand, evaporative emission control systems are generally employed in internal combustion engines. One of such evaporative emission control systems is shown in FIG. 1, which comprises a fuel tank 51, a canister in which evaporative fuel is adsorbed and stored, an evaporative fuel passage 53 connecting between the fuel tank 51 and the canister 52, and a two-way valve 54 arranged across the evaporative fuel passage 53.
With this arrangement, when the pressure of evaporative fuel generated in the fuel tank 51 reaches a predetermined level, the evaporative fuel forcibly opens a positive pressure valve of the two-way valve 54 to flow therethrough into the canister 52 where it is adsorbed by an adsorbent 55 and thus stored therein.
During rotation of the engine, vacuum developed in an intake pipe, not shown, causes evaporative fuel to be drawn from the canister 52 together with fresh air introduced into the canister 52 through an air inlet 56, into a purging pipe 57 to be supplied to engine cylinders, not shown, through the intake pipe.
When the fuel tank 51 is cooled due to ambient temperature etc. so that negative pressure increases within the fuel tank 51, a negative pressure valve of the two-way valve 54 is opened to return evaporative fuel stored in the canister 52 into the fuel tank 51. In this manner, the two-way valve 54 regulates the pressure within the fuel tank 51 at a constant level to thereby prevent evaporative fuel generated in the fuel tank 51 from being emitted into the atmosphere.
However, in the evaporative emission control system constructed as above, when evaporative fuel is generated in large quantities within the fuel tank 51, the amount of evaporative fuel adsorbed by the adsorbent 55 within the canister 52 increases correspondingly, and sometimes the adsorbent 55 can be saturated with evaporative fuel adsorbed therein. This can cause deterioration of the adsorbent 55, and even cause evaporative fuel from the fuel tank 51 to pass through the canister 52 into the atmosphere without being adsorbed by the adsorbent 55, resulting in contamination of the atmosphere.
If the purging amount of evaporative fuel is increased by a purging control system, not shown, in order to prevent the above inconveniences, a large amount of evaporative fuel is supplied to the engine cylinders, resulting in fluctuations in the air-fuel ratio of a mixture supplied to the engine and hence degraded exhaust emission characteristics as well as degraded driveability.